A lithographic process is one that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically done by imaging the patterning device onto a layer of radiation-sensitive material (resist) provided on the substrate by way of an optical system (e.g. a projection lens). Stepping and/or scanning movements can be involved, to repeat the pattern at successive target portions across the substrate. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An important property of interest is critical dimension (CD). It is important that structures are formed with accurate critical dimension control over the whole substrate (e.g. wafer). A key parameter in order to control the critical dimension during the lithographic process is the position of the substrate relative to the focal plane of the lithographic apparatus (which may also be known as the “focus setting”). In particular, control of the focus setting must be carefully maintained during exposure of the substrate. This may be achieved by controlling the focus characteristics of the projection lens, and or by controlling the position of the substrate such that it is kept close to the focal plane of the projection lens during exposure of the substrate.
Typically, focus settings are determined by performing measurements on one or more focus targets. The focus targets are positioned on a patterning device (e.g. reticle) and are patterned onto the substrate using a lithographic step. Typically the patterning device also comprises structures associated with the pattern of the product (e.g. an IC), said structures referred to as “product structures”. After patterning the product structures and focus targets are present on the substrate. The focus target are measured (for example in a metrology or inspection apparatus) and the focus setting is determined. The determined focus setting is representative for a certain deviation between a reference and an actual focus setting during exposure of the substrate. Knowledge of the focus setting may be used to correct the lithographic apparatus focus in order to enhance performance of the lithographic process. This correction may be achieved by adjusting an optical element within the projection lens of the lithographic apparatus or by adjusting the position and/or orientation of the substrate with respect to the focal plane of the projection lens of the lithographic apparatus.
The determined focus setting is representative for a certain deviation between a desired and an actual focus setting during exposure of the substrate. Knowledge of the focus setting may be used to correct the lithographic apparatus focus in order to enhance performance of the lithographic process. This correction may be achieved by adjusting an optical element within the projection lens of the lithographic apparatus or by adjusting the position and/or orientation of the substrate with respect to the focal plane of the projection lens of the lithographic apparatus.
However, the focus targets take up space on the substrate. This directly reduces the number of product structures that can be placed on a substrate, which is undesirable. Additionally, the positioning and features of the focus targets may cause interference with nearby product structures, thereby potentially degrading the quality of these product structures.
Further, in order to determine the focus setting it is necessary to carry out test and calibration procedures in addition to the measurements themselves. Furthermore, such measurements have to be carried out using a metrology or inspection apparatus. The substrates under measurement are therefore delayed during the measurement process, which proportionally increases production time and thereby throughput of the lithographic apparatus.
The known method measures the total focus setting for a substrate, which includes all focus error sources. Therefore, it may be difficult to identify the root cause of any defects or focus error sources whilst using the above method and apparatus. The known method does not distinguish between different sources of focus errors (e.g. errors caused by the lithographic apparatus or errors caused by the lithographic process). Accordingly, identifying and correcting focus errors and their source may take a significant amount of time.
Typically the used focus targets are placed on a product reticle (a reticle comprising product structures) and comprise diffractive structures having a pitch smaller than the pitch of the product structures. After these focus targets are patterned (exposed in resist), the focus setting can be determined from diffraction based measurements. Basically the focus setting is reconstructed from the observed diffraction pattern. This method of measuring a focus setting is commonly referred to as “diffraction based focus” (DBF) measurement. Its focus targets are referred to as diffraction based focus targets (eg “DBF targets).
The fact that the focus targets and product structures are patterned during the same lithographic process is essential. The focus targets are exposed at exactly the same conditions as the product structures (same dose settings, illumination mode, lens settings, stage characteristics etc.). The measured focus settings are hence representative for focus behavior of the lithographic apparatus during production, e.g. the determined focus setting is relevant for both focus target and product structures.
The described diffraction based method to measure a focus setting was found to be less successful when the thickness of the resist was chosen to be very thin. This is for example the case when adopting an Extreme Ultra-Violet (EUV) lithographic process for which the resist must be very thin to prevent a too strong absorption gradient throughout the resist stack. In case of a thin resist stack, for example thinner than 50-100 nm a diffraction based method will suffer as the radiation used to perform the diffraction based metrology increasingly becomes reflected from structures underlying the resist pattern. In addition the required pitch of the DBF targets scales with the product structure pitch. Sub-resolution pitches for a EUV process, as required for the DBF targets, will become increasingly challenging in view of manufacturability of the DBF targets on the reticle.
The invention proposes a solution to measure a focus setting representative for the lithographic process during production when adopting a thin resist and/or high resolution lithographic process (for example EUV or a low kl-DUV process).